.content{
    width: 640px;
    height: 640px;
    margin: 100px auto;
    position: relative;
}
.btn{
    width: 100%;
    height: 100%;
    position: absolute;
    left: 0;
    top: 0;
    background-color: rgba(0, 0, 0, 0.3);
    z-index: 2; /* 为了让遮罩层在蛇跟苹果的上面*/
}
.btn button{ /* 将button上面有input表单的元素，不需要*/
    background: none;
    border: none;
    background-size: 100% 100%;/* 引用图片用的大小样式*/
    cursor: pointer;/* 手图标样式*/
    outline: none;
    /* 放上去会有线的样式 */
    position: absolute;
    left: 50%;
    top: 50%;
}
.startBtn button{
    width: 200px;
    height: 80px;
    background-image: url(../images/startGame.png);
    margin-left: -70px;
    margin-top: -40px;
}

.pauseBtn{
    display: none;
}
.pauseBtn button{
    width: 70px;
    height: 70px;
    background-image: url(../images/pauseGame.png);
    margin-left: -35px;
    margin-top: -35px;
}

#snakeWrap{
    width: 600px;
    height: 600px;
    background: #225675;
    border: 20px solid #7dd9ff;
    position: relative;
}

/* #snakeWrap div{
    width: 20px;
    height: 20px;
} */

.snakeHead{
    background-image: url(../images/snake.png);
    background-size: cover;
    /* cover覆盖，将背景等比例缩放完全覆盖背景 */
}
.snakeBody{
    background-color: #9ddbb1;
    border-radius: 50%;
}
.food{
    background-image: url(../images/apple.png);
    background-size: cover;
}